I'm having the exact same problem with an Asus RT-N16 running the same build. The only information that I've yet to find on it doesn't seem to help at all:
Subject: Re: WRT310N Disconnect problem
Date sent: 13 Oct 2010, 17:51 GMT-04
OK, I think I have found out somthing to fix this… I don't know exactly what I did, but one of the firewall settings must have changed somthing… which though, I do not know, but my ping now does not go above 6ms while in game on my 360.
Heres what I set:
RIPv1 & v2 Disabled
DHCP Routes unchecked
Spanning-Tree Protocol unchecked
Use Internal DNS was unchecked
Respond to ICMP ping selected
Allow multicast All
NAT loopback selected
Enable SYN cookies selected
This is with the current final release. Does this help shed light on the issue? I will try messing with all of the firewall settings until I can pinpoint which one made my ping so much better/=.
It'd be really great to find a solution to this. The only reason I bought two wireless gigabit routers (TP-Link WR1043nd is the other) was to have great speed for writing large backups across the network. If I'm limited to 20mhz with it's 130Mbps max link speed, then the second gigabit router is a waste.
For what it's worth, I just changed over to Tomato today because of a similar problem in DD-WRT (here the pings are terrible and the transfer is god-awful slow; there, it works great … 10 hours, or 4 hours, or 2 hours, or 30 minutes before completely disconnecting.)